An ionic liquid is a salt in the liquid state, and is entirely comprised of ionic composition. Because the ionic liquid remains liquid at room temperature or a lower temperature (−97° C.˜100° C.), the ionic liquid is described as a room temperature molten salt or a low temperature molten salt, and also as a liquid organic salt. There are many types of ionic liquids, and according to different organic cations, ionic liquids can be divided into quaternary ammonium salts, quaternary phosphonium salts, nitrogen heterocyclic onium salts, etc., such as nitrogen heterocyclic type ionic liquids including imidazolium onium salts, pyridinium onium salts, piperidinium salts, pyrrolidine salts, etc. There are various types of anions which could constitute ionic liquids, wherein inorganic anions comprise F−, Cl−, Br−, I−, NO3−, CO32−, PF6−, BF4−, C2O42−, SO42−, PO43−, Al2Cl7−, etc., while organic anions comprise CH3COO—, CF3SO3—, C4H9SO3—, CF3COO−, N(FSO2)2−, N(CF3SO2)2−, N(C2F5SO2)2−, N(C4F9SO2)2−. N[(CF3SO2)(C4F9SO2)]−, C(CF3SO2)3−, etc. Theoretically speaking, there exist more than 1018 kinds of ionic liquids. Structures of cations and anions of several common imine based ionic liquids are as follows:

In the 1970s, American scientist John S. Wilks applied ionic liquids into a battery system for the first time. Since the 1990s, extensive research has been conducted on applying ionic liquids into lithium ion secondary batteries, and the feasibility and superiority of ionic liquids acting as electrolyte solvents of lithium ion secondary batteries have gained increasing recognition and attention from industry experts. Compared with the current widely used organic solvents with carbonate ester, ionic liquids acting as the electrolyte solvents of lithium ion secondary batteries have the following advantages: (1) The liquid temperature region of ionic liquids is wider than that of a conventional solvent. For example, dimethyl carbonate (DMC) is widely used in lithium ion secondary batteries as an electrolyte solvent, but it has only a narrow liquid temperature region of 2˜90° C. Furthermore, for most ionic liquids, the maximum liquid temperature may reach about 300° C. (their decomposition temperature). Some other ionic liquids may have much wider liquid temperature region from −70° C. to 400° C., which have greatly expanded the temperature scope of the application of lithium ion secondary batteries (for example, extending to higher temperature application). (2) As ionic substances, ionic liquids have good dissolving ability, and their concentration is adjustable. Corresponding ionic liquids can dissolve several common lithium salts, such as LiPF6, LiBF4, LiCF3SO3, LiN(SO2CF3)2, etc., and a high solute concentration can be achieved, which could meet the requirements of lithium ion concentration in electrolytes when using a lithium ion secondary battery as a power battery. (3) Ionic liquids have good ionic conductivity, and their ionic conductivity could be up to 1˜10 mS/cm in the absence of lithium salts. (4) Ionic liquids have both a high thermal stability and a high chemical stability. For example, the thermal decomposition temperature of most ionic liquids would be more than 400° C., and in normal cases, ionic liquids would not react with common electrode materials of lithium ion secondary batteries, such as LiFePO4, LiCoO2, Li4Ti5O12, graphite, etc. (5) Ionic liquids have no noticeable vapor pressure even at temperatures more than 100° C. When the battery runs in high temperature environment, deformation in battery due to the extremely high pressure would not occur. For example, it is difficult for a ‘bulge’ phenomenon to occur when ionic liquids are applied in a soft-packaging battery using an aluminum-plastic composite membrane. (6) Ionic liquids have no flash point, but their fire points are high. Some ionic liquids are not flammable even if exposed to open flames. Carbonate solvents that are currently used in secondary batteries are flammable and combustible; because of that, there is a safety threat when applying such carbonate solvents into a lithium ion secondary battery. By comparison, ionic liquids can be applied to electrolytes, and are expected to solve the safety issues of the lithium ion secondary batteries.
At present, ionic liquids used as electrolyte solvents in lithium ion secondary batteries are mainly quaternary ammonium salts, pyridinium salts, pyrrolidine salts, imidazolium onium salts, and pyridinium onium salts with their anions being BF4−, PF6−, CF3SO3−, N(CF3SO2)2−. Different combination of anions and cations have great influence on physical and chemical properties of ionic liquid based electrolyte, and even directly impact the performance of lithium ion secondary batteries. In recent years, many studies have shown that those ionic liquids whose anions are iminium ions tend to have lower melting points, and their combination with a variety of cations would form molten salts whose melting points are lower than 0° C. All these advantages broaden the scope of the choice of the cations, making quaternary ammonium cations, piperidine cations, and pyrrolidine cations with greater electrochemical stability suitable for application in lithium ion secondary battery systems. For example, N-methyl-N-butyl-piperidinium bis (trifluoromethylsulfonyl) imide [PP13-TFSI] with a melting point of −18° C. has an excellent performance when used in a Li/LiCoO2 battery system: the specific capacity of the positive active material can reach 150 mAh/g, the coulombic efficiency can reach 100%, and there is no obvious decay after ten weeks of circulation (XU Jin-qiang, etc., [J]chemistry journal, 63(18): 1733); Zheng Honghe research group from Suzhou University finds that when the N,N,N-trimethyl-N-hexyl-bis(trifluoromethylsulfonyl) imide quaternary ammonium salt is applied to the lithium ion secondary battery whose negative active material is hard carbon, even under high temperature of 80° C., the battery could also discharge and charge normally, and intercalation/deintercalation behavior of ionic liquid cation in hard carbon will not occur. That research group believes that the combination between such ionic liquids and hard carbon will have prospective applications (RSC Adv., 2012,2,4904 4912).
The traditionally used processing technology of ionic liquids, taking a quaternary ammonium salt as an example, is an alkylated reaction between a tertiary amine and an alkyl halide, and its reaction is shown below:R1R2R3N+R4X→[R1R2R3R4N]+X−  (1)
For example, tributyl methyl iodide ammonium can be obtained from the reaction of tributyl tertiary amine and iodomethane:(C4H9)3N+CH3I→[(C4H9)3NCH3]+I−  (2)
During the preparation of a quaternary ammonium salt having at least one methyl substituent on nitrogen element, the dimethyl sulfate could also be used as an alkylating agent, as shown below:R1R2R3N+(CH3)2SO4→[R1R2R3NCH3]+CH3SO4−  (3)
It is easier for a tertiary amine to react with dimethyl sulfate, and such reaction has a high yield. But using dimethyl sulfate also has disadvantages such as high toxicity, and it may cost cancer. The biggest disadvantage of the above-mentioned technique is that it can only prepare certain quaternary ammonium salts. For example, for quaternization reaction of the alkyl halide, this process can only prepare the quaternary ammonium salts whose anion is Cl−, Br−, or I−; for quaternization reaction of dimethyl sulfate, this process can only prepare quaternary ammonium salts whose anion is CH3SO4−. To prepare other quaternary ammonium salts whose anion is another ion, ion-exchange reactions such as those shown in formula (4) and formula (5) could be used:[R1R2R3R4N]+X−+H+A−→[R1R2R3R4N]+A−+H+X−  (4)[R1R2R3R4N]+X−+M+A−→[R1R2R3R4N]+A−+M+X−  (5)
For example, when preparing the quaternary ammonium salt [R1R2R3R4P]22+SO42− whose anion is SO42−, generally, a chlorinated quaternary ammonium salt would be firstly synthesized via the formula (1) reaction; and then, making the quaternary ammonium chloride to react with sulfuric acid via the reaction of formula (4), and removing hydrochloric acid by taking advantage of the volatile feature of hydrochloric acid, therefore making the reaction (4) equilibrium go to right, thus making ion-exchange to maximum extent. And for another example, when preparing a quaternary ammonium salt [R1R2R3R4P]+BF4− whose anion is BF4−, similarly, corresponding quaternary ammonium halides salt is firstly synthesized via formula (1) reaction; and then, via formula (5), the quaternary ammonium halides salt and the metal inorganic salt such as NaBF4 react in the organic solvent such as acetone, since the metal halide has a low solubility in the organic solvent, halide ions would precipitate from the solution in the form of precipitation, thus ion-exchange would be realized. Obviously, formula (4) and formula (5) are equilibrium reactions, which mean they do not always go to completion, which would inevitably lead to halide ion residue in final products. Even if silver salts such as AgBF4 are used to facilitate reaction (5) being carried out in aqueous solution, which may react completely, the cost would be prohibitively expensive.
On the one hand, halogen anions such as Cl−, Br−, and I− have poor stability, and tend to be oxidized to release poisonous and corrosive halogen substance, which restrict its application scope; On the other hand, after extensive research, it has been found that when the anion is chosen from one of the following ions group of F−, NO3−, CO32−, PF6−, BF4−, C2O42−, SO42−, PO43−, Al2Cl7−, CH3COO−, CF3SO3−, C4H9SO3−, CF3COO−, N(CF3SO2)2−, N(FSO2)2−, N(C2F5SO2)2−, N(C4F9SO2)2−, N[(CF3SO2)(C4F9SO2)]−, C(CF3SO2)3−, etc., quaternary ammonium salts normally have certain features that quaternary ammonium halides salts usually do not possess, such as low melting points, high conductivity, low viscosity and strong hydrophobicity etc., therefore they have a wider scope of potential application. For this reason, to develop a new preparation process of these special quaternary ammonium salts is particularly important.
U.S. Pat. No. 4,892,944 describes a method of preparing a quaternary ammonium/phosphonium salt using dimethyl carbonate as an alkylating agent. The method includes two steps, in the first step, tertiary amine/phosphine reacts with dimethyl carbonate to generate a quaternary ammonium/phosphonium methyl carbonate; in the second step, the quaternary ammonium/phosphonium methyl carbonate react with an acid to release methanol and carbon dioxide, and obtain a quaternary ammonium/phosphonium salt, the anion species of the quaternary ammonium/phosphonium salt are determined by the acids being used, and reaction equations are as follows:R1R2R3N(P)+Me2CO3→[R1R2R3N(P)Me]+MeCO3−  (6)[R1R2R3N(P)Me]+MeCO3−+H+A−→[R1R2R3N(P)Me]+A−+MeOH+CO2  (7)
One feature of this method is that, the anions of the obtained quaternary ammonium/phosphonium salts derive from anions of various acids, which will not be limited by quaternary ammonium/phosphonium agents, and the materials of anions can be select from a variety of ranges. However, the reactants have to be limited within a tertiary amine or a tertiary phosphine. Only the tertiary amine or the tertiary phosphine could be alkylated by dimethyl carbonate to generate corresponding quaternary ammonium/phosphonium salt; however, ammonia (NH3), the primary amine, the secondary amine or hydrogen phosphide (PH3), the primary phosphine and the secondary phosphine could not be alkylated by dimethyl carnonate, and therefore could not get quaternary ammonium/phosphonium cations.
Both relevant Chinese patents (No. CN200510061094.4, application date 2005.10.10; No. CN200710008626.7, application date 2007.2.14) disclose that a kind of dialkyl carbonate react with an amine (ammonium) salt at a suitable temperature and a pressure (50° C.˜300° C., 0.5 MPa˜50 Mpa, 4˜12 h) to generate a quaternary ammonium salt; both patents take the carbonate ester as an alkylating agent, and the hydrogen of the amine salt is substituted by methyl in reaction and thus quaternary ammonium salt is obtained. However, these two technical solutions also have great differences, and one of the main differences lies in the use of catalysts. The technical scheme of Patent No. CN200510061094.4 needs to use the catalyst selected from a metallic compound, a non-metallic compound, its mixture or an ionic liquid; therefore, how to separate the product from the catalyst still remains an issue after completion of the reaction, and it is difficult to ensure the high-purity of the product. Meanwhile, the technical scheme disclosed in Patent No. CN200710008626.7 does not use catalysts, and does not need subsequent complicated separation process, its operating process is relatively simple, while its product purity improves greatly. In that way, it is more useful to some applications which have much higher product purity requirements. However, these two methods all emphasize the synthesis of corresponding quaternary ammonium salt from an amine (ammonia) salt, namely, taking the products after neutralization reaction between an amine (ammonia) and an acid as reactants, such as NH4+L−, RNH3+L−, R1R2NH2+L−, R1R2R3NH+L−.